First of all, relax. If the only thing you owe is $480, be happy that there isn't any case pending against you in juvenile court. This appears to be just an infraction, like a traffic ticket.
Continue to do what you are supposed to be doing. Listen to grandma, obey her rules, refrain from drug or alcohol use, and go to school everyday. Have grandma take you back to the court where the ticket was issued and ask to see the judge to see if you can have the fine converted to community service (like volunteer work at a nonprofit agency). If they see that you are behaving and trying to do well, most judges and/or district attorneys will give someone in your situation a break.
I am sorry for your situation . It's unfortunate that your parents were not there for you . Speaking with a social worker might help with this. Maybe you can get court to convert fine into community service.
You should go to the court where the ticket was issued and request the court clerk to place the matter on calendar. Then show up at the date and time that the matter is calendared and request community service in lieu of paying a fine.
The above answer is for general information only and is based on the information you posted. Every case is fact dependent, so to get a thorough analysis of your situation, you will need to consult face to face with an attorney licensed to practice in the jurisdiction where the incident took place. Do not conclusively rely on any information posted online when deciding what to do about your case.